The present invention relates to a process for producing pulverous selenium from raw selenium by distillation, drying and grinding.
The anode slime from copper electrolysis contains considerable amounts of selenium, which can be recovered in the form of raw selenium after the removal of copper and nickel. The copper is removed first from the anode slime by leaching the slime in sulfuric acid and by blowing air through the solution to dissolve the copper, whereafter the nickel can be removed from the slime by sulfating it by means of strong sulfuric acid. The selenium is roasted from the slime as selenium dioxide gas by means of sulfuric acid at a high temperature; the gas is then reduced to elemental selenium. This raw selenium is washed and finally filtered, whereby a raw material suitable for the production of pure pulverous selenium is obtained.
The raw selenium is purified by distillation at a high temperature, and for this purpose the damp raw selenium obtained from the filter must first be dried. The molten selenium obtained from the distillation is granulated in water and must thereafter be re-dried before grinding in a ball mill.
According to a currently used process, raw selenium is purified as follows:
In order to facilitate heat drying of raw selenium, pressurized air is blown through the damp raw selenium cake in the filter. The selenium detached from the filter falls into boxes situated below. Owing to the air blast the surface of the cake is already at this stage so dry that the material produces dust when falling into the boxes, and thus selenium passes into the atmosphere of the working space. The maximum allowable concentration is 0.1 mg/m.sub.3. Currently this concentration is surpassed when the filter is being emptied, and the workers must use breathing masks.
The boxes in which the raw selenium is collected from the filter are placed in a heat cabinet one on top of the other. Air is blown through the cabinet and is heated by electric resistors. At first, when the evaporation of dampness is great, the temperature remains below 100.degree. C., but ultimately it rises to 100-110.degree. C. The drying in the heat cabinet lasts approximately one week. Raw selenium is crystalline and resists heating without sintering.
The dried raw selenium is batched into tanks. The batching is performed by hand. It is also fed by hand into the distillation retort. During these operations, selenium dust inevitably passes into the atmosphere of the working space.
The distillation is performed at 680.degree. C. in a retort which is heated using light naphtha. The distilled selenium is granulated from liquid to solid state by dripping it into cold water. The produced raw selenium is amorphous and hardly resists any heating without sintering.
The distilled selenium accumulates in a tank provided with a mesh bottom. This tank is inside a larger tank, through which cold, pure water is run in such a way that during the distillation the water level is high and covers the selenium accumulated on the mesh bottom. After the distillation, all of the water is let out. The raw selenium is allowed to drip overnight on the mesh bottom to facilitate the final drying in the heat cabinet.
The distilled selenium which has remained overnight on the mesh bottom is shoveled or scooped into four mesh-bottomed boxes. The boxes are placed in the heat cabinet one on top of the other. Air is blown through the cabinet, and the air is heated to some extent by means of electric resistors. The set value of the temperature is 45.degree. C. If the temperature rises above this, the selenium granules are sintered firmly together.
The drying time in the heat cabinet approx. 24 hours, whereafter the selenium is taken into a tank to be transferred to the grinding apparatus.
The dry, granular material is fed, for grinding, into a porcelain-lined ball mill together with steel grinding balls. The necessary grinding time is 24 hours. The ground selenium is scooped, together with the balls, into the tank in which it is transported to the sieving station. When the ball mill is being emptied, the selenium is again in the form of dust-producing powder, which produces a great deal of dust, not only when the mill is being emptied but also during the operations following (when being fed into the sieve, during the sieving, and during packing), thereby causing excess concentrations in the surrounding atmosphere.
As regards the grinding, it should also be mentioned that the steel balls cause the iron content in the product to rise greatly during the grinding. This is naturally not desirable considering the purity of the product. The results of an experiment are given below:
______________________________________ Sample % by weight Fe A B ______________________________________ Distilled selenium in granules 0.003 0.002 After grinding 0.011 After grinding and sieving 0.010 ______________________________________
The porcelain lining may also increase the amount of impurities in the product.
The sieving is performed using a rapid sieve, the mesh of the sieve being 400 mm. All of the sieved product passes a 50-mesh sieve and 80% of it passes a 100-mesh sieve.
The object of the present invention is to provide a process for the production of pulverous selenium with a higher capacity and with a smaller number of dust-producing operations than so far, the product of the process being a selenium powder which contains less iron and resists heat better without sintering than the pulverous seleniums produced by current processes. A further object is to provide a process by which the particle size of the selenium can be regulated even during the production.